1. Field of the Invention
This invention relates to a process for removing an oxidizable substance or a reducible substance, which are present, for example, in a liquid industrial waste, by decomposing the oxidizable substance or reducible substance in the presence of a metal oxide or hydroxide. The invention further relates to a composite comprising an organic cation exchanger and a metal oxide or hydroxide supported on the cation exchanger, which is suitable for the process for removal of the oxidizable substance or reducible substance, and to a process for producing the composite.
2. Description of the Related Art
Metal oxides and hydroxides as catalysts are used in a finely divided particulate form. The finely divided particulates have poor working and handling characteristics, and therefore, are usually used as shaped articles or in a form supported on a carrier.
For example, a cation exchange membrane made of a fluorinated polymer for electrolysis of an aqueous solution of an electrolyte such as an alkali metal chloride is described in Japanese Unexamined Patent Publication No. 5739185, U.S. Pat. No. 4,661,218 and European Patent Application No. 66,102. The cation exchange membrane has a gas and liquid permeable porous layer formed thereon to reduce the cell voltage, which layer is made from a powder of, for example, a metal oxide by a process wherein the metal oxide powder is mixed with a binder in a liquid medium, and the mixture is applied onto the membrane and heat-pressed on the membrane. The resulting cation exchange membrane is a composite composed of a cation exchange membrane of a fluorinated polymer and a porous layer which is made from a metal oxide powder and physically bonded to the membrane.
Processes for decomposing reducible substances (COD ingredients) wherein a liquid containing reducible substances is placed in contact with a metal oxide catalyst in the presence of an oxidizing agent such as a hypochlorite in a fluidized form have been proposed in Japanese Unexamined Patent Publication No. 52-41453, 52-105651 and 52-23860. In these proposed processes, the catalyst is removed and recovered after they are used for the decomposition, to avoid a pollution nuisance and reduce the treating cost. However, since the catalyst is finely divided, it is difficult to filter and a filtering apparatus of a large capacity is required.
To solve the above-mentioned problems of the metal oxide catalyst in a finely divided form, it has been proposed to use a metal oxide catalyst in a form supported on a carrier or in a shaped article form made by using a binder, for example, in Japanese Unexamined Patent Publication No. 52-128648, 49-37465 and 55-27075. More specifically, a catalyst composed of a nickel oxide, manganese oxide or cobalt oxide, which is supported on a carrier by using a chlorine- and/or fluorine-containing resin binder, is described in Japanese Unexamined Patent Publication No. 55-27075. This catalyst is made by supporting a salt of nickel, manganese or cobalt on a carrier by using the resin binder; curing the resin binder; and then the supported metal salt is treated with an oxidizing agent. The catalyst supported on a carrier has good and handling characteristics, but has problems such that the catalyst and the resin binder are liable to be separated during the use of the catalyst with the result of a pollution nuisance due to suspended substances and the metal catalyst ingredient. Further, where the solution to be treated contains suspended substances or ions capable of readily forming insoluble or slightly soluble substances, such as a calcium ion and a magnesium ion, problems arise such that the suspended substances or a precipitate formed from the ion are deposited on the surface of the catalyst with the result of a drastic reduction of the catalytic activity.
As an example of the process for decomposing reducible substances in a solution by treating the solution with a metal oxide, a process has been proposed in Japanese Unexamined Patent Publication No. 63-130141 wherein a hypochlorite is decomposed by treating the hypochlorite-containing solution with a catalyst composed of a platinum-group metal oxide alone or of a platinum-group metal oxide supported on an inorganic carrier such as titanium or ceramics. Where this catalyst in a powder form is used in a suspended bed or fluidized bed, it is difficult to prevent the catalyst from flowing out from the suspended bed or fluidized bed. If the catalyst is in the form of a shaped article, the catalyst cannot be used in a suspended bed or fluidized bed. Further, a baking treatment must be carried out at a temperature exceeding 300.degree. C. for supporting the platinum-group metal catalyst on the inorganic carrier.
It has been proposed to use a catalyst in the form of a solution or suspension of a salt or oxide of nickel or copper in Japanese Unexamined Patent Publication No. 56-65633 and 60-71086. To avoid a pollution nuisance and reducing the catalyst cost, the catalyst must be separated and recovered from a liquid which has been treated with the catalyst, and occasionally the recovered catalyst must be subjected to a regeneration treatment. However, the catalyst is in a finely divided form and is difficult to recover in an industrial scale without the use of an apparatus having a large capacity.
Various catalysts have been proposed which are in the form of a shaped article or supported on a carrier, for example, in Japanese Unexamined Patent Publication No. 61-149240 and 56-108587, UK Patent Application No. 2,059,793, and U.S. Pat. Nos. 4,430,315 and 4,442,227. More specifically, a pelleted catalyst composition comprising a catalytically active ingredient such as nickel oxide or cobalt oxide embedded in an organic resin matrix is described in UK Patent Application No. 2,059,793. This pelleted catalyst composition is made by intimately dispersed a powdery catalytically active ingredient in a powdery organic resin matrix such as a polyolefin or a halogenated polyolefin; forming the intimately mixed powdery composition into pellets by compacting them; and then sintering the pellets at or about the softening temperature of the organic resin matrix. In this pelleted catalyst composition, the catalytically active ingredient is physically compacted with the organic resin, and therefore, the bonding force is weak and the catalyst composition cannot be used in a suspended bed or a fluidized bed. Even where the catalyst composition is used in a fixed bed, the catalyst is separated from the pellets when used for a long period of time, which leads to loss of catalyst, clogging of pipe lines and a pollution nuisance due to suspended substances and the separated catalyst. If a solution to be treated with the catalyst composition contains solid ingredients, the solid ingredients must be removed prior to the treatment with the catalyst composition. Further, the process of the preparation of the catalyst composition is complicated and lengthy, and only the surface portion of the pelleted catalyst participates in the reaction.
In Japanese Unexamined Patent Publication No. 60-71085, a method of decomposing hydrogen peroxide in a liquid by using a composite comprised of a palladium catalyst supported on an anion exchange resin such as a styrene-divinylbenzene copolymer-based anion exchange resin. This composite has an enhanced effective surface area, but a poor thermal resistance and chemical resistance because of the hydrocarbon polymer. Since the composite is used in a liquid containing hydrogen peroxide, the catalyst activity is reduced at a high rate.